Abdullah bin Umar reported,
Ibn Umar came out to us, and we were hoping he would tell us a hadith that would delight us. A man rushed up to him and said, "Abu Abdul-Rahman, what do you say about fighting during civil strife, given that Allah 
has said, 'And fight them until there is no more fitnah'?"Ibn Umar replied, "Woe to you, do you know what fitnah is? The Messenger of Allah ﷺ used to fight the polytheists, and entering into their religion was the fitnah. It is not like your fighting one another over kingship."
Ahmad 5690
Sahihحدثنا هشام بن سعيد حدثنا خالد يعني الطحان حدثنا بيان عن وبرة عن ابن جبير يعني سعيدا
عن ابن عمر قال
خرج إلينا ابن عمر ونحن نرجو أن يحدثنا بحديث يعجبنا فبدرنا إليه رجل فقال يا أبا عبد الرحمن ما تقول في القتال في الفتنة فإن الله عز وجل قال {وقاتلوهم حتى لا تكون فتنة} قال ويحك أتدري ما الفتنة إنما كان رسول الله ﷺ يقاتل المشركين وكان الدخول في دينهم فتنة وليس بقتالكم على الملك
Its isnad is sahih. Hisham bin Saad Talaqani, a teacher of Ahmad, we established his reliability earlier at 4981, and we noted there a discrepancy among the manuscripts of Tarikh al-Kabir and Manaqib Ahmad by Ibn Jawzi regarding his father's name, whether it was Saeed or Saad, and we concluded that it was Saad because the three original manuscripts agreed on that. But here it appears as Saeed, again by agreement of the three original manuscripts, so perhaps this is the correct reading, Allah willing.Khalid Tahhan is Khalid bin Abdullah bin Abdul-Rahman bin Yazid Tahhan Wasiti, whose reliability we established earlier at 554, and we add here Ahmad's statement, "Khalid Tahhan was reliable and upright in his religion." Abu Hatim said, "Reliable, sound in hadith," and he gave his biography in Tarikh al-Kabir volume 2, part 1, page 147. The hadith is given at length at 5381.